FIGS. 1 and 2 show block constructions of a general digital remote control system. In the Figures the reference numeral 31 designates a transmitting circuit including a key input read circuit 11, a code modulation circuit 12, a timing generator 13, and an oscillator 14. The reference numeral 32 designates a receiving circuit including a preamplifier 18, and a remote control signal demodulation circuit 19. The reference numeral 33 designates a light emitting diode or other light emitting element. The reference numeral 34 designates a photo diode or other light receiving element. The reference numeral 10 designates a key matrix for inputting information to the key input read circuit 11 of the transmitting circuit 31. The reference numeral 15 designates a driver circuit comprising a transistor which receives a coded signal from the code modulation circuit 12 of the transmitting circuit 31 and causes a corresponding current to flow through the light emitting element 33. The reference numeral 16 designates a modulated light information beam transmitted from the light emitting element 33 to the light receiving element 34.
In such a system, the instruction to be transmitted is input to the transmitting circuit 31 by the key matrix 10, encoded by the transmitting circuit 31, modulated and transmitted in a light signal 16 by the light emitting diode 33. The transmitted light signal 16 is received by the photo diode 34, and demodulated by the receiving circuit 32 to decode the instruction.
FIG. 3 shows a transmission format in such a transmission system which has been already developed by the present inventor. The discrimination of one bit information "0" and "1" is conducted by detecting the intervals 41 and 42 between two sequential pulses as shown in FIG. 3. That is, the short time interval 41 from the rising edge of one pulse to the rising edge of the next pulse (in FIG. 3(a)) corresponds to a bit "0", and the long time interval 42 of that FIG. 3(b) corresponds to a bit "1". Several "0" and "1" bits are combined to constitute a word as shown in FIG. 4, and various instructions are distinguished from each other by decoding the data code of this word. In the example of FIG. 4, one word 5 comprises a six bit construction, and in this figure the data word 5 is "010000". Herein, the code 6 designates the repetition period of the word 5.
In this transmission system, however, the time length of the word depends on the number of "0" or "1" bits in a word, and this results in difficulty in the interpretation of data because of unawareness of the length of a specific word at the receiving side. Furthermore, as shown in FIG. 5, when a noise signal occurs between two pulses which represent the bit "1", this bit "1" is erroneously judged as "00" at the receiving side, leading to a malfunction. This causes a fatal defect in such a remote control system.
In order to avoid interference between remote control systems, systems are distinguished from each other by a custom code for distinguishing the apparatus to be controlled comprising an initial two bits of a transmission data code, while the other subsequent four bits constitute an instruction code for operating the apparatus to be controlled, as in the example of FIG. 4. However, in the remote control field, various remote controls having various bit constructions are adopted, and therefore there is a possibility that interferences may arise which prevent the system from being used when criteria for judging the "0" or "1" bits are similar to each other such that bit numbers of different systems may undesirably coincide with each other.